Regulatory T cells, or “Tregs” which encompass CD4+ and CD8+ Foxp3+ Treg cells and CD45RClow Tregs are fundamental in controlling various immune responses in that Tregs can rapidly suppress the activity of other immune cells. In particular, Tregs are crucial for maintaining tolerance by downregulating undesired immune responses to self and non-self antigens. For instance, Treg defects have been discovered in patients with multiple sclerosis (MS), type I diabetes (TID), psoriasis, myasthenia gravis (MG) and other autoimmune diseases. Similar links may also exist for atopy and allergic diseases. For all these diseases reports exist pointing to a reduced in vitro immune suppression of the patient's Treg cells. This has led to an increasing interest in the possibility of using Tregs in immunotherapy to treat or prevent autoimmune diseases, allergies and transplantation-related complications, such as graft rejection or graft-versus-host disease (GvHD).
For instance, organ transplantation has seen very significant improvements in both the prevention and treatment of acute rejection, but subclinical episodes and chronic graft dysfunction still heavily impact medium and long-term graft survival (1). Emerging therapeutic strategies, among them tolerance induction to donor antigens, are moving to the clinical stage after years of experimental model work (2, 3). Among natural mechanisms and tolerance inductive strategies, the use of different types of regulatory cells, including different types of CD4+ Tregs, are among the most promising ones (4). The uses of CD8+ regulatory T cells (CD8+ Tregs) have been highlighted in recent years by ourselves, and others, in the transplantation field, but also in other pathological situations (5-8).
Hence, there is a particular need for methods useful for generating and expanding Treg cells with high degree of purity, and simultaneously CD4+ and CD8+ Tregs, preferably without CD4+ and CD8+ effector T cells in order to obtain such a purified population of Treg cells which is particularly of interest in the fields of autoimmunity, allergy, transplantation, treatment with therapeutic protein and gene therapy, to avoid degradation of self or therapeutic molecules/tissues by the immune system.
Interleukin-34 (IL34) was identified in 2008 (9). Studies showed that IL34 shares homology with M-CSF and they act through a common receptor, CD115, also called CSF-1R, (9) expressed on the cell surface of monocytes, and in the brain through a newly described receptor, Receptor-type Protein-tyrosine Phosphatase ζ (PTP-ζ) (10). However, studies have demonstrated that IL34 and M-CSF display distinct biological activity and signal activation (11), in part due to their differing spatial and temporal expression (12). Up to now IL34 function has been mainly linked with the survival and function of monocytes and macrophages (osteoclasts, microglia) (12). IL34 protein expression in resting cells has been observed in keratinocytes, hair follicles, neurons, proximal renal tubule cells and seminiferous tubule germ cells (12), and also in heart, brain, lung, liver, kidney, spleen, thymus, testicles, ovaries, prostate, colon, small intestine, spleen red pulp and osteoclasts (9). So far, IL34 has not been linked to the effects on immune function of DCs or T cells (12).
More recently, IL34 was shown to induce the differentiation of human monocytes into immunosuppressive macrophages (also referred as IL34-Mφ) since said IL34-differenciated macrophages suppressed TCR-dependent T cell proliferation (13), to induce transplantation tolerance (13) and also to induce IL-17-producing effector T helper cells, called Th17 cells (14). However, IL34-differenciated macrophages have never been shown to be useful for generating and expanding Treg from peripheral blood mononuclear cells (PBMCs).